It is estimated that 80% of significant hearing impairment in the U.S. is sensorineural or "nerve" deafness that usually arises from hair cell loss or damage. Deficits in the sensory hair cells that convert sound into electrical activity int he inner ear have been considered irreversible, because in the human ear the production of hair cells ceases before birth. However, hair cells are produced in the ears of cold-blooded animals postembryonically; in some, millions of new hair cells could be replaced through regenerative proliferation that could be evoked by trauma. It was most recently learned that new hair bundles could appear in the balance organs of mammals, including humans, after antibiotic poisoning. This application for renewal of a project in its 13th year proposes to continue investigations into the cellular processes that underlie hair cell production, hair cell death, and the regenerate replacement of hair cells lost during postembryonic life. The discoveries of hair bundle replacement in mammals and trauma-evoked regenerative proliferation that can be increased by growth factor treatments in mammalian epithelia have further strengthened the central thesis, that the obstacles to effective regeneration of hair cells in the ears of mammals are not likely to remain insurmountable, if control of postembryonic production and development of hair cells can be understood at cellular and molecular levels. The project will use cell culture, RT-PCR, in situ hybridization, hybridoma development, time-lapse video microscopy, laser microbeam ablations of individual cells, assays for apoptosis, and patch-clamp recording. The objectives are; 1) identification of the events and factors that will enhance proliferation of hair cell progenitors and the differentiation of hair cells, 2) the development of methods and reagents for accelerated investigations of regeneration, 3) the characterization of programmed and trauma-induced cell death in the ear, and 4) investigations of the functional properties of regenerated and recovering hair cells. The information sought is essential for understanding the capacities of regeneration and self-repair in hair cell epithelia. The goals of the work directly pertain to possible recovery from sensorineural hearing loss and balance disorders, and to improved understanding of the development of normal and abnormal auditory and vestibular function in human ears.